1. Field of the Invention
Exemplary aspects of the present invention generally relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device employing a pressure adjustment mechanism that adjusts pressure of a pressure roller against a fixing roller, and an image forming apparatus incorporating the fixing device.
2. Description of the Background Art
There is increasing demand for energy-efficient, high-productivity image forming apparatuses such as printers, copiers, facsimile machines, and multifunctional machines including at least two of these functions. In order to accommodate such conflicting demand, typically, optimizing the heating efficiency of a fixing device used in the image forming apparatus, which fixes an unfixed toner image onto a recording medium, is of critical importance.
There are various ways of fixing the unfixed toner image onto the recording medium. For example, the image forming apparatuses can form an unfixed toner image directly or indirectly onto the recording medium, typically a recording sheet or paper including photosensitive paper and electrostatic recording paper, using such image forming methods as electrophotography, electrostatic recording, and magnetic recording. Here, forming the unfixed toner image indirectly on the recording medium means forming the unfixed toner image initially on a transfer member and then transferring it onto the recording medium.
To fix the unfixed toner image on the recording medium, a fixing device using a contact-type fixing method is widely used.
Generally, a known fixing device using a heating roller method is equipped with a hollow heating roller, also known as a fixing roller, and a pressure roller disposed opposite the heating roller. A heat source (e.g., a halogen lamp) is disposed inside the heating roller to heat the heating roller. The heating roller and the pressing roller meet and press against each other, together defining a fixing nip or simply nip therebetween. The recording medium bearing an unfixed toner image is guided into and through the fixing nip, where heat and pressure are applied to the unfixed toner image on the recording medium to fuse and fix the unfixed toner image onto the recording medium.
In order to reliably convey the recording medium bearing the unfixed toner image to the fixing nip, a sheet guide is provided substantially at the beginning of the fixing nip to guide the recording medium in a more stable manner. However, if the sheet guide is not properly shaped or properly positioned, pressure is not applied evenly in the fixing nip, thereby causing undesirable creasing of the recording medium. In particular, in recent image forming apparatuses, the recording medium is most likely to be conveyed upward in the vertical direction (vertical conveyance) during the transfer and fixing processes. The orientation of the recording medium is not stable during the vertical conveyance. Thus, the sheet guide needs to be properly disposed for reliable conveyance.
While reliable conveyance of the recording medium to the fixing nip is a necessity, it contributes only indirectly to meeting consumer demand for energy efficiency and higher productivity. To satisfy at least the latter requirement directly, various methods for shortening rise time, for example, have been proposed. For example, increasing the width of the fixing nip can increase fixing speed and reduce the rise time, thus increasing productivity.
However, although advantageous, this approach has a drawback in that having a wide nip width requires more heat, increasing power consumption. Moreover, the recording medium tends to crease more easily in the wide fixing nip. In addition, if such a nip width is not changeable, it is difficult to obtain a desired fixing nip width in accordance with different types and thicknesses of the recording media sheets.
In view of the above, JP-2008-102409-A proposes to adjust the width of the fixing nip in accordance with the type and thickness of the recording media sheets as well as the heating conditions of the fixing device. In this configuration, the fixing device employs a pressure adjusting mechanism to change the position of the pressure roller to adjust an amount of engagement of the pressure roller against the fixing roller to obtain an optimum nip width.
Although advantageous, there is a drawback in that when the pressure roller is moved to adjust the nip width while the position of the sheet guide is fixed (to a housing of the fixing device or the like), the position of the sheet guide relative to the fixing nip is changed. In other words, the position of the sheet guide does not change with the movement of the pressure roller. As a result, the recording medium is not reliably conveyed to the fixing nip.
In view of the foregoing, a device capable of stably conveying the recording medium even when the position of the pressure roller changes to obtain an optimum nip width is needed, in order to provide an energy-efficient, high-productivity image forming apparatus.